This invention relates generally to an electrochemical cell for use in a gas detector unit, and more particularly, to a cell for sensing reducible gases including, for example, NO.sub.2, Cl.sub.2, O.sub.3, ClO.sub.2 and O.sub.2. This is achieved by the use of gold as the sensing electrode in combination with a material selected from the group of iridium and ruthenium as the counterelectrode, and preferably iridium. For three electrode sensors wherein a reference electrode is utilized, preferably the material will be selected which is similar to the counterelectrode material. However, the material may be selected from platinum, iridium and ruthenium. The effect of this combination of materials utilized in the electrodes of the sensor of the invention is a more stable continuously operating cell, not subject to the usual degradation of prior art cells. This invention is an improvement over the inventions described in U.S. Pat. Nos. 3,992,267, issued Nov. 16, 1976; 3,824,167, issued July 16, 1974; 3,776,832 issued Dec. 4, 1973 and 3,909,386, issued Sept. 30, 1975. Each of these patents is incorporated by reference in its entirety herein.
With ever increasing concern about pollution of our environment and our increasingly sophisticated knowledge with respect to the presence of polluting materials in the environment, attempts have been made to develop systems which will protect us by warning of increases in the concentration of certain substances in the ambient to a level which is dangerous to our existence. One such device which has been developed in recent years is a gas sensor for sensing the presence of such gases adjacent a work area, for example. As will be appreciated, it is important that such sensors continue to operate over a period of time so that certain enclosed areas are protected, for example, such as mines where certain levels of concentration of gases may cause death if exposure is for a specific period of time. It is important, also, from a manpower standpoint that the instruments need not be continuously attended to because of any rapid deterioration of the sensing capacity thereof.
However, electrochemical gas sensors, for example, are subject to certain limitations over a period of time merely because of the chemical nature in which they operate, in the sense that the sensing capacity degrades. For example, in the past, in electrochemical sensors for detecting reducible gases such as NO.sub.2 and Cl.sub.2, where both the sensing and counterelectrode are comprised of gold diffusion electrodes, and wherein sulfuric acid is used as the electrolyte, a large irreversible polarization of the gold counterelectrode occurs during detection. This polarization causes sluggish response characteristics during detection. Whereas such sluggish responses may not always be of substantial importance, there are instances where the rapid sensing of the presence of a gas is important. Another example of such degradation over a period of time involves the use of platinum as the counterelectrode material. In such an arrangement, there is gradual degradation in the sensor specificity over a period of time with a gradual increase of, for example, an interferring carbon monoxide sensitivity signal.